In optical information communication, an optical fiber, which is a transmission medium, has been required to have increased transmission capacity in accordance with an increase in communication traffic. Particularly, a long-distance transmission optical fiber employing WDM (wavelength division multiplexing) has been keenly required to meet such a requirement. It is expected that the communication traffic will steadily increase. In view of the circumstances, an increase in the transmission capacity of the optical fiber is an urgent object to be attained.
It is necessary to increase power of signal light to be supplied to the optical fiber so as to increase the transmission capacity of the optical fiber. However, in a case where the power of the signal light to be supplied to the optical fiber is increased, power density in a core of the optical fiber increases. This causes problems such as production of a nonlinear optical effect and generation of fiber fuse. It is therefore necessary to suppress an increase in the power density in the core by increasing an effective core area of the optical fiber, in order to increase the power of the signal light, to be supplied to the optical fiber, so that the transmission capacity of the optical fiber is increased.
As a single mode fiber having an increased effective core area, there have been known optical fibers disclosed in Patent Literatures 1 and 2.
Patent Literature 1 discloses a technique of increasing an effective core area of a single mode fiber having a depressed refractive index profile to 120 μm2 or more. Further, Patent Literature 2 discloses a technique of increasing an effective core area of a single mode fiber, having a core including a first core layer (high refractive index), a second core layer (low refractive index), and a third core layer (intermediate refractive index), to 130 μm2 or more. The single mode fibers disclosed in Patent Literatures 1 and 2 have no mode dispersion. In this regard, each of the single mode fibers is advantageous to its high transmission capacity.
However, the single mode fibers disclosed in Patent Literatures 1 and 2 each have the following problem. That is, in a case where bending loss is to be limited to a given value (for example, a value required under a standard) or less, it is not possible to freely increase the effective core area.
That is, it is necessary to cut off a second order mode (LP11 mode) so as to realize a single mode fiber which propagates only a fundamental mode (LP01 mode). In order to cut off the second order mode, it is necessary that a core radius a, a refractive index n1, and a refractive index n2, each of which is a parameter specifying a refractive index profile, satisfy Expression (1). Note here that V denotes a normalized frequency, and λ denotes a wavelength.
                    V        =                                                            2                ⁢                π                            λ                        ⁢                          an              1                        ⁢                                          2                ⁢                                  (                                                                                    n                        1                        2                                            -                                              n                        2                        2                                                                                    2                      ⁢                                              n                        1                                                                              )                                                              ≤          2.405                                    (        1        )            
However, it is not possible to freely increase the core radius a in a case where the core radius a satisfies Expression (1). Accordingly, it is not possible to freely increase the effective core area. Note that, in a case where the refractive index n1 of a core of the single mode fiber is decreased, the core radius a can be increased. Meanwhile, decrease in the refractive index n1 of the core increases the bending loss. Therefore, in a case where a standard or the like determines an upper limit of the bending loss, the core radius a is inevitably limited by Expression (1).
On the other hand, a core radius of a multimode fiber is not limited by Expression (1). This allows the multimode fiber to have an effective core area larger than that of the single mode fiber. However, according to the multimode fiber, occurrence of a mode dispersion cannot be avoided. The mode dispersion causes a decrease in transmission capacity in proportion to a transmission distance. Therefore, it is important to suppress the mode dispersion so as to realize a multimode fiber which can carry out long-distance transmission with large transmission capacity.
A two-mode optical fiber is a multimode fiber which propagates only the fundamental mode (LP01 mode) and the second order mode (LP11 mode) and whose mode dispersion is most easily suppressed. Some two-mode optical fibers have positive mode dispersions Δτp, the others have negative mode dispersions Δτn. By combining the two-mode optical fiber having the positive mode dispersion flip with the two-mode optical fiber having the negative mode dispersion Δτn, an optical transmission line having a mode dispersion compensation function can be configured. Specifically, the mode dispersion compensation function is realized by setting (i) a length Lp of the two-mode optical fiber having the positive mode dispersion Δτp and (ii) a length Ln of the two-mode optical fiber having the negative mode dispersion Δτn so as to satisfy a proportional expression: Δτp:Δτn=Ln:Lp.